Chromatography is the separation of a mixture of compounds (solutes) into separate components. This separation permits the composition of all or part of the mixture to be determined. In gas chromatography, a gas chromatograph (commonly called a “GC”) is utilized to separate and determine the quantities of components of a gas mixture. A gas chromatograph performs these functions by taking a sample of the gas mixture to be analyzed and injecting it into a carrier gas stream, such as helium or hydrogen, which then carries the gas sample through one or more tubes (referred to as columns) that are packed with a very fine particulate material. Each of the particles of this material are coated with a film from liquid that controls the rate at which the different components of the gas sample are absorbed and de-absorbed by the particulate material. This rate of absorption and de-absorption also varies relative to each of the different components. Because of this differing rate of absorption and de-absorption, certain gas molecules related to one type or component of gas will exit the column more quickly than some of the other components will. This process of separation of components permits a detector located at the end of the column to quantify the amount of a particular component that is present in the mixture.
There are a variety of detectors used in gas chromatographs, with thermal conductivity detectors (“TCDs”) being the most common. TCDs can be built using any of a number of types of temperature sensing elements, including but not limited to negative temperature coefficient thermistors (“NTC thermistors”), or platinum RTD's, etc. These temperature sensing elements have a resistance value that varies as a function of temperature.
A conventional gas chromatograph typically uses a pair of thermistors in a bridge circuit to quantify the components in a gas sample. A typical bridge circuit includes a reference thermistor and a sense thermistor connected into arms of the bridge circuit. The reference thermistor is connected into a carrier gas line that carries a carrier gas to an injection valve where a sample gas may be injected into the carrier gas. The sense thermistor is connected into an outlet line coming from the column. When only carrier gas flows through the outlet line, the bridge is balanced (with an output of zero volts) because the same gas with the same thermoconductivity is flowing by the reference thermistor and the sense thermistor. When carrier gas together with an eluting component of the sample gas flows through the outlet line, the bridge becomes unbalanced due to the difference in thermoconductivity between the gas flowing by the reference thermistor and the gas flowing by the sense thermistor. This imbalance is used to provide a measure the quantity of the eluting component.